1. Field of the Invention
This invention relates generally with reducing self noise in sonar systems. More particularly, the invention relates to reducing self noise from water flow vibrations and machinery noise in underwater acoustic systems.
2. Description of the Prior Art
A sonar array works by detecting the incoming pressure fluctuations due to the sound a target makes in the water. The pressure responses of each individual sonar array element are converted into electrical signals which are added together coherently (i.e., the phase of the signals with respect to each other must be taken into account) to give the array output.
The term "self noise" as used with sonar arrays describes the noise in the output signal of the array due to vibrations in the sonar array structure or the platform upon which the array is mounted. The sonar array is comprised of multiple sonar elements. Each sonar element is connected to an array mounting plate by an isolation mount. The isolation mount is a spring-like device, typically fabricated from a cylindrical section of a somewhat compliant material.
Low self noise is desirable because it enables the sonar to detect low level incoming signals. This in turn increases the acquisition range for a specified target. Assuming all electrical sources of self noise have been eliminated or minimized, mechanical sources are the next sources to consider.
For underwater vehicles, an acoustic array is typically mounted on the front or nose of the craft. As the craft moves through the water, the water flow travels around the nose and at some point along the shell of the craft, the water flow turns from laminar to turbulent. The vibrations due to this transition are a source of noise whereby energy from the turbulence is transferred through the nose structure to the array, exciting the array elements through two paths. The first path is through the tip of the nose into the fluid and enters the elements via the pressure response. The second math is through the array mounting plate and the element's isolation mount.
Experiments indicate the dominant path that the vibrational energy follows (i.e., through the water or through the array mounting) depends on the type of sonar beam that is formed. For beams formed from a single element or from a few elements, the water path is usually dominant. For beams formed from many elements, the path through the array plate and element isolation mount is dominant. However, when vibrations through the element's mounting have been reduced, as with the two stage trilaminar isolation mount, reducing vibration transmission through the fluid path provides significant additional reductions for both single element and multi-element beams.
Several methods have been proposed in the industry for reducing self noise. One technique is to design the contour of the nose shell to delay the onset of turbulent flow to a point substantially downstream from the nose. This moves the source of vibration further back along the shell away from the array.
Another technique is to design the shell with large impedance mismatches which reduce the transmission down the shell. Sonar array windows that wrap around the nose shell can provide some damping of vibrations in the shell as can damping material applied directly to the inside of the shell. Shells made of composite construction have also been tested. Array element mounting techniques that reduce the vibration transmitted through the element mounts are the standard way of reducing sonar self noise.
Self NOise REduction (SNORE) rods have been tested in the industry to reduce the diffraction of sound around the torpedo nose shell. However, SNORE rods have been largely ineffective because diffraction of sound is not presently a major cause of self noise. Reducing self noise caused by direct vibration transmission through the fluid path has not been addressed.
In arrays presently known in the art, a solid ring which is part of the shell surrounds the array. In this arrangement, the vibrations are transferred down the shell and can get into the array by radiating from the ring and coupling through the water path into the sonar elements. Alternatively, the vibrations can get into the elements via the vibration response of each element because the elements sit on a plate which is caused to vibrate by the turbulence.
The industry has attempted to address the self noise problem in underwater sonar devices. However, with the exception of the SNORE rod concept, which dealt with the diffraction of sound around the nose shell and not at the more critical problem of radiation from the nose shell, no attempt has been made to reduce vibration transmission in the fluid coupling path. Thus, self noise reduction techniques are needed which address the problem of self noise caused by a vibration transmission through the fluid path.